New readings from the European satellite Envisat suggest that this year's southern hemisphere ozone hole may be one of the largest on record.

New readings from the European satellite Envisat suggest that this year’s southern hemisphere ozone hole may be one of the largest on record.

The hole covers an area of 10 million sq km (four million sq miles) – approximately the same size as Europe.

It is expected to continue expanding for two to three weeks.

There have been signs over the last two years that damage to the ozone layer has reduced, but a full recovery is not expected until around 2050.

European eye

The data comes from the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (Sciamachy) on board Envisat, launched in 2002 and operated by the European Space Agency (Esa).

They show that the Antarctic ozone hole was larger in mid-August this year than at the same period in any year since 2000.

“We expect that depletion will continue for another month or so,” Geir Braathen, senior scientific officer with the World Meteorological Organization (WMO), told the BBC News website.

“Typically, it reaches a maximum around mid-September, though the exact date varies from year to year.

“The biggest holes on record came in 2000 and 2003. This year’s may be as big, but we will have to wait another two or three weeks to be sure.”

Chlorine catalyst

Ozone depletion is caused by the release of various human-produced substances including chlorofluorocarbons (CFCs) and methyl bromide.

An unstable molecule consisting of three oxygen atoms, ozone is constantly being produced and destroyed in the stratosphere, where it forms a protective layer against harmful ultra-violet radiation from the Sun.

Naturally, across the globe, production and destruction are roughly in equilibrium, though ozone concentrations above the poles rise each summer and decline during the winter months.

This seasonal variation is especially pronounced in the Antarctic, where a huge vortex spins above the continent, isolating its air from the rest of the atmosphere and preventing ozone from migrating in.

In the 1920s, the natural order began to change with the invention of CFCs, subsequently used widely for refrigeration and in various industrial applications.

In cold conditions, polar stratospheric clouds form which contain ice crystals; compounds derived from CFCs lodge on their surfaces.

The polar sunlight falling onto these compounds transforms them further, into substances which catalyse the destruction of ozone.

This process is much more active in winter, which adds to the seasonal dips in ozone concentration at both poles.

Annual changes

As well as varying from summer to winter, polar ozone levels also show changes from year to year.

“The Arctic is far more variable than the Antarctic,” commented Geir Braathen, “but there is inter-annual variation in the Antarctic which is down to variations in weather.

“There is something called the quasi-biennial oscillation, or QBO, where the wind above the Equator blows either east or west.

“That changes with a period of 26 months, and has ramifications for the Antarctic vortex – which is why we see inter-annual changes in ozone depletion there.”

Two years ago researchers produced the first evidence that damage to the ozone layer is slowing down; globally, they showed, destruction continues, but at a slower rate than before.

That is down to the Montreal Protocol, established in 1987, which has limited production and use of CFCs and related substances.

But the indications are that the ozone layer will not be back to its pre-industrial condition for at least another 50 years. BBC

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